1. Field of the Invention
The present invention relates to an apparatus for detecting particles that may be stuck on the surfaces of substrates, for example, reticles and masks used for producing printed circuit patterns on semiconductor wafers, such as in the production of LSI (large-scale integrated) product wafers, or substrates that are used in liquid crystal displays, and more particularly, to an optical system for specifying a size of the particles and a location on a substrate where the particles are adhered.
2. Description of Related Art
Referring to FIG. 2, a conventional apparatus for detecting particles is disclosed. A substrate 1, for example, a substrate provided with a circuit pattern formed on a surface thereof, can be aligned horizontally on an inspection stage (not shown) and provided with a pellicle frame 2 on an upper surface thereof. The substrate 1 is adapted to be slidably moved in both the directions shown by the arrow X and arrow Y to enable positional alignment on the inspection stage.
An optical system 3, which can include an He-Ne laser cavity 4 for transmitting a laser beam L, having an appointed angle of polarization, is provided with a beam expander 5. The expanded laser beam L contacts a beam scanning mirror 6, for example, a galvanomirror, that can revolve in the direction indicated by the arrows U-V, to enable the laser beam L to be scanned through a lens 7. The lens 7 permits the laser beam L to be incident upon the surface of the substrate to be inspected. Thus, a linear scanning in the direction shown by the arrow X will occur so that the laser beam, generated from the laser cavity 4, will be irradiated on the substrate to be inspected from an appointed or predetermined angle relative to the vertical plane with a reciprocation of a linear scanning within an appointed or predetermined range in the direction shown by the arrow X.
A detector optical system 8 is arranged at both ends of the substrate in the X-direction and is also inclined to the plane of the substrate. Each of these optical systems includes a collecting lens 9, a fixed slit aperture plate 11, having a long and slender slit 10 in the vertical direction for limiting an incidence of a reflected and scattered beam R which has been reflected and scattered from the particles on the surface of the substrate to be inspected. An optical detector 12 is capable of receiving the laser beam L which has been incident upon the substrate and reflected or scattered from the particles. The optical detector 12 can comprise, for example, a photomultiplier tube and the like for detecting the reflected scattered beam R.
In operation, the laser beam L is incident upon the substrate at the predetermined angle and linearly moves across the inspection stage in the direction shown by the arrow Y, while reciprocally and linearly scanning within the predetermined range in the direction shown by the arrow X. As a result, a reflected and a scattered beam R is reflected and scattered from the particles on the substrate to be inspected and is detected by the optical detector 12. If any particles exist on the surface of the reciprocating substrate to be inspected and is contacted by the laser beam L, it is capable of scattering the laser beam at random in all directions, which will impact the intensity of light measured by the optical detector 12. Correlating the incident of detection with the spatial position of the substrate will indicate the existence of a particle on the surface of that substrate at that location.
As can be appreciated, a close correlation between the actual position of the laser beam L on the substrate surface is necessary for accurately determining the position of any particle. If any deviation is provided in the relationship between the angle of swing of the beam scanning mirror 6, which reciprocally scans the laser beam L transmitted by the laser cavity 4, and the angle of the swing signal put out from the beam scanning mirror 6 on account of any drift in the electrical system resulting from an influence by temperature, or an aging of the circuit parts, or a variation in tolerances of the circuit parts and the like, then a lapse of time occurs, which will correspondingly create an error in determining the accurate position of any particle.
Referring to FIG. 3, when the electrical system does not exhibit any drift and is accurately correlated with the position of the laser beam, then a linear relationship is maintained between the position of the particles obtained by calculation on the basis of the scanning position signals (angle of swing signals) and the actual position of particles. This relationship is shown as a straight line I in FIG. 3. As can be determined, the calculated position of particles corresponding, for example, to particle A and particle B will amount to SA and SB, respectively, and accordingly the positions of the particles A and B can be detected and accurately reproduced for subsequent visual examination purposes. However, if any drift in the electrical system or other error occurs, a deviation, D, can be produced in the relationship between the calculated positions of the particles and the actual position of the particles. Thus, the calculated position of particles corresponding to particles A and B can apparently occur at positions SA' and SB' by incorporating the deviational error D. It is possible that temperature changes can be minimized by the incorporation of a temperature compensation feature into the circuit driving the beam scanning mirror 6, but it is extremely difficult to account for aging and tolerance changes of circuit components and the like that can occur over a period of time with the use of the instrument.
Thus, there is a demand in the prior art to provide a relatively improved economical particle detecting system.